In a power system, electric generators supply electricity to loads via a network of transmission lines and distribution lines. In the supplying of electricity, all the electric generators synchronously operate; frequency of the power system decreases when the load on the power system is above electricity supplied from the electric generators, and increases when the load on the power system is below the electricity. Each electric generator, by use of a governor, therefore increases output when the frequency decreases, and decreases output when the frequency increases (governor-free operation).
Furthermore, when fluctuation in the load cannot be absorbed fully by the governor-free operation, the electric generators perform load frequency control to adjust its output according to an output adjustment signal received from an electricity supply control center. However, because of their slow response, the electric generators have a problem of low following capability to output adjustment signals.
As an exemplary solution to the problem, Patent Literature (PTL) 1 discloses a technique for performing load frequency control by utilizing a dispersed power system, in which high responsiveness of secondary batteries is principally leveraged. With this, according to PTL 1, responsiveness of load frequency control is increased and thereby frequency stability is improved.